1. Field
The present application relates to amplifiers. In particular the present application relates to dynamic bias adjustment for an amplifier operating in the envelope tracking mode.
2. Description of Related Art
Although nonlinear amplifiers can exhibit higher efficiency than linear amplifiers, until recently nonlinear power amplifiers were undesirable for use with RF signals produced by linear modulation schemes. A technique known as “envelope tracking” (ET) was developed that allows use of linear amplifiers to approach the efficiency of non-linear power amplifiers with RF signals produced by linear modulation schemes (e.g. where it is important to maintain relative variation within an envelope of an RF signal). By dynamically adjusting a DC bias voltage at a drain terminal of an output transistor of a power amplifier in a manner that roughly follows a time varying envelope of the RF signal, a signal produced by a linear modulation scheme can be amplified by a power amplifier without undesirable envelope distortion. Effectively, the technique of envelope tracking shifts a burden of linearity away from the power amplifier to an ETPS (envelope tracking power supply) that is connected to the drain terminal of the output transistor of the nonlinear power amplifier.